While some support the introduction of civil partnerships for heterosexuals, others warn of a "dark" and "cynical" attempt by Tory MPs to destroy the bill.

David Cameron addresses guests at the gay pride reception in the garden at 10 Downing Street, in central London on June 16, 2010. Photograph: Getty Images.

After devoting last week to an esoteric debate over Europe, this week is set to be another in which the Conservative Party demonstrates its increasingly tenuous relationship with the modern world. The gay marriage bill is back in the Commons for its report stage and David Cameron is likely to face a revolt on the scale of that in February when 136 Conservative MPs opposed the legislation.

Before the main vote tomorrow, MPs will vote tonight on an amendment[3] tabled by former Tory minister Tim Loughton that would extend civil partnerships to heterosexual couples. Loughton, who opposes same-sex marriage, insists that the amendment has been submitted in good faith, but the government is briefing that it is an attempt to "wreck" the legislation. (It's worth pausing to note the oddity of Tory MPs opposing gay marriage, which won't "undermine" the institution of marriage, while supporting heterosexual partnerships, which certainly will.) It has warned that the change could delay the passage of the bill by up to two years and cost the government an additional £4bn in pension liabilities. On the Today programme this morning, equalities minister Maria Miller said:

Look, I want to be seeing marriages being undertaken under this new bill as early as next summer and to actually put in at this stage such a fundamental change I believe risks that and it risks significant delay and I think those that are supporting it need to be very aware of that.

Miller's words were a warning to Labour, which has pledged to support the amendment on the grounds of equality. It has dismissed[4] the government's warnings as "farcical", noting that the supposed size of the "price tag" has grown from £3bn to £4bn in five days. Some Labour MPs also believe that ministers may be preparing to use the passage of the amendment as a convenient excuse to abandon the bill. But other equality campaigners echo the government's concerns.

Despite long supporting the introduction of civil partnerships for heterosexuals, the Lib Dems are set to vote against the amendment for fear that it will wreck the bill. Lynne Featherstone, the former equalities minister, said: "The people pushing these changes are not those with records of supporting equality and marriage rules that accommodate a diversity of couples.

"The proposals are coming from those who are avowed and determined opponents of equal marriage. Have they suddenly become converts to the cause of equality?

"Given their public statements I fear what is at work here is rather darker and more cynical – a deliberate attempt to wreck the legislation."

In addition, the gay rights group Stonewall has said that it is "anxious about anything that could delay this much needed change in the law to bring about marriage equality".

But other campaigners, most notably Peter Tatchell, have urged MPs to support the amendment to correct a long standing injustice. Asked if he was concerned that Loughton and other Tory MPs were proposing it simply to "wreck" the bill, he said[5]: "Yes, I am concerned but we should do the right thing, regardless of their shabby motives. Equality for all. You can't fault that."

The outcome is now likely to rest on whether the government can persuade Labour that its warnings are sincere and that it should reconsider its position.